General aviation aircraft are typically equipped with one or more altimeters that determine altitude by measuring changes in barometric pressure. Changes in barometric pressure are determined by a diaphragm that is responsive to atmospheric pressure. The response of the diaphragm to changes in barometric pressure is used to drive an indicator, such as a needle, relative to a scale on a dial face to thereby indicate altitude.
In order to provide accurate altitude readings, an altimeter must be adjusted to the local barometric pressure on the ground at a given location. To adjust the altimeter to the local barometric pressure, a small window in the dial face, referred to as a Kolsman window, exposes a barometric pressure scale. A knob is provided for adjusting the barometric pressure scale to the desired setting. By turning the knob one way or the other, the desired barometric pressure can be aligned with a mark on the dial face thereby setting the altimeter.
The altimeter should be adjusted to the local barometric pressure at takeoff and landing. For example, prior to takeoff a pilot will typically receive an altimeter setting (the local barometric pressure) over the radio from a ground controller and adjust the altimeter accordingly. Once set to the local barometric pressure, the altimeter should indicate the elevation of the airfield provided that the aircraft is at the same reference level used to establish the altimeter setting. Once in flight, the pilot will periodically obtain current altimeter settings en route, and adjust the altimeter accordingly. Periodically adjusting the altimeter is important, for example, when flying from an area of high pressure into an area of low pressure. If the altimeter is not adjusted to the new lower local barometric pressure, the aircraft will be closer to the surface than the altimeter indicates. In this regards, an inch change in the altimeter setting equals approximately 1,000 feet of altitude. When approaching an airfield for landing, the pilot will again receive a local altimeter setting and adjust the altimeter to the local barometric pressure at the landing airfield.
Adjusting an altimeter to the local barometric pressure is critical to ensuring the most accurate altitude reading possible. Indeed, in the history of aviation, many mishaps have occurred due to inaccurate and/or improperly adjusted altimeters. Many factors can contribute to a pilot setting an altimeter inaccurately. For example, the pilot may misunderstand the altimeter setting dictated by the ground controller over the radio and thereby set the altimeter to an incorrect barometric pressure. The pilot may inadvertently adjust the altimeter to a inaccurate barometric pressure by misreading the barometric pressure scale through the Kolsman window. Further, while in flight, turbulence may prevent the pilot from accurately reading and/or adjusting the altimeter to the proper pressure.
For pilots with any visual impairment due to age, such as myopia or cycloplegia, difficulty adjusting the altimeter is further compounded, particularly in turbulence or in low light conditions. As a large percentage of general aviation pilots are over the age of 40, the age at which cyclopegia commonly starts occurring, difficulty in viewing the Kolsman window is a prevalent problem. As such, a long felt yet unresolved need exists for an altimeter having a Kolsman window that is easy to read.